Class / spec is irrelevant for everything except soloing edge raids, of which there are normally only a few. That is, in any given patch, the majority of the old raids are soloable by any spec, a select few current / old raids aren't soloable no matter the spec, and it's the remaining few raids, and frequently even specific encounters that can be soloed by some specs, but not others. The thing is, when it comes to these few raids which can be soloed by some specs, but not others, we are always talking about top level current raiding gear for the specs that can technically solo them. So, unless you have access to that top level gear, it doesn't make sense to even wonder about what these specs are, that won't be a limiting factor.

With all that said, traditionally, the strongest soloing specs are tanks. Among tanks, historically, the strongest were the DKs.

In terms of racial abilities, I will note the goblin's rocket jump. It does help with soloing, when you try to do things which would normally require two warm bodies. Shadowmeld was useful for soloing some time ago as well (eg, for doing the first boss of BT in Cata). But these are just gimmicks, I wouldn't choose a race based on just that, the impact is small.

1. Blood DK - The ultimate solo class! Will help with soloing old raid achievements.
2. Prot Paladin - For the same reasons as above.
3. Rogue - Many advantages in the PvP achievements (BG achievements, etc).

Are you Alliance or Horde? If you're Alliance and you want to focus on achievements with some PVP then go human. You'll save yourself a lot of time grinding out whatever reps you aren't exalted with, and you get the pvp trinket racial for any PVP that you do.

By solo'ing do you mean you want to solo fairly recent content? Then you'll want a tank spec, and to be a blood DK if you are really serious about it. Most classes can solo a lot of older things once you get an expansion away from it, but for more recent things tanks will be your best bet.

"This is no swaggering askari, no Idi Amin Dada, heavyweight boxing champion of the King's African Rifles, nor some wide shouldered, medal-strewn Nigerian general. This is an altogether more dangerous dictator - an intellectual, a spitefull African Robespierre who has outlasted them all." - The Fear: Robert Mugabe and the martyrdom of Zimbabwe, Peter Godwin.

Are you Alliance or Horde? If you're Alliance and you want to focus on achievements with some PVP then go human. You'll save yourself a lot of time grinding out whatever reps you aren't exalted with, and you get the pvp trinket racial for any PVP that you do.

By solo'ing do you mean you want to solo fairly recent content? Then you'll want a tank spec, and to be a blood DK if you are really serious about it. Most classes can solo a lot of older things once you get an expansion away from it, but for more recent things tanks will be your best bet.

Well good idea for the Human part. But in WoD does the racial stay with the new racial remake ?
When i mean solo'ing when i was playing my hunter it was fairly easy to kill stuff while my pet takes the damage i was bursting it down with DPS . But for me the hunter class seems overpopulated and i sometimes like to feel a bit more away from the crowd . DK seems like a option , but for me the rune systems looks kinda complicated ( i haven't played one )

My goal with this is to be an achivement player ( achivement whore if you wanna call it ) while everyone spends time raiding and just afking on the capital city . i wanna explore the world and quest since i haven't done the cata old zones revamp and wanna do outlands before WoD hit . want to chose something unique and worth using to do this goal